The invention relates generally to radio frequency identification (RFID) systems and, more particularly, to techniques for reducing harmful interference effects in RFID systems.
A radio frequency identification (RFID) system is a system that uses radio frequency transponders (i.e., tags) to identify items-of-interest. Each radio frequency transponder is attached to a corresponding item and includes information identifying that item. When an identification needs to be made, a radio frequency reader unit (i.e., an interrogator) is used to excite (i.e., interrogate) the transponder on the item which then transmits an identification signal (including the identification information for the item) back to the reader unit. The reader unit then uses the identification information received from the transponder to perform any of a number of different RFID applications. For example, the identification information can be used to perform functions such as asset management, inventory tracking, access control, and others.
In an RFID system, a problem arises when one or more external interference signals are present within the frequency band of the system during an interrogation operation. Such interference will often cause misidentification of items-of-interest and faulty reporting in the RFID system. Increasingly, such interference is being caused by other RFID readers and/or transponders located near the system-of-interest that operate within the same or a similar frequency band. One method for preventing such interference between RFID systems is to use a master control unit that allocates an operating window for each RFID system to operate within so that, for example, multiple systems do not perform interrogation operations at the same time. Such an approach, however, requires that each of the similarly located systems be under the control of the same host or master, which is not always possible. In addition, such an approach requires that additional wiring and control elements be provided to implement the centralized control. Such systems also generally require reprogramming every time an RFID system is added or removed from the area. Furthermore, it is not always simple to determine when one system will negatively interfere with another from a centralized location. That is, the operational requirements of nearby systems will often differ and, therefore, the effects of interference within a particular system will best be made within the system itself.
Therefore, there is a need for a method and apparatus for reducing the negative effects of in-band interference in an RFID system. The method and apparatus will preferably allow multiple RFID systems to be located near one another with minimal interference effects between systems. Preferably, the method and apparatus will not require a master control unit to control the timing of the multiple neighboring systems. In addition, the method and apparatus should be capable of automatically adapting to changing system conditions and a varying spectral environment.
The present invention relates to a radio frequency identification (RFID) system that is capable of operating in close proximity to other RFID systems that utilize the same or a similar frequency range, with reduced interference effects. Before initiating an interrogation operation, a reader unit in the system senses the spectral environment surrounding the reader to determine whether there are any potentially harmful interference signals present. If no such signals are present, the reader initiates the interrogation operation immediately. If potentially harmful interference signals are present, the reader unit waits a quasi-random delay period before checking the spectral environment again.
The quasi-random period of time is selected so that another RFID system will normally be able to complete its interrogation operation before the expiration of the quasi-random delay period. After the quasi-random delay period has ended, the reader again checks the spectral environment and, if necessary, waits another quasi-random delay period. Eventually, the reader unit will normally be able to perform its interrogation operation within a relatively clean spectral environment, thereby reducing or eliminating negative interference effects within the reader. If, however, the reader is not able to obtain a clear channel after a number of attempts, the reader can attempt to operate in spite of the interference. The reader can also decide to report the situation to a host for troubleshooting purposes.